1. Field of the Invention
The invention relates to a bright-field transmitted-light lighting device for microscopes.
2. Description of Related Art
Bright-field transmitted-light lighting devices for microscopes are generall , designed in accordance with Kohler lighting conditions. such that the light source provides a uniform illumination of the object field and of the entrance pupil of the objective. The resolving power of this microscope arrangement is dependent both upon the aperture of the objective employed and the lighting aperture. In the case of relatively large lighting apertures, the attainable resolving power of the microscope system increases, while the image contrast decreases. For optimal use of the objective employed, the lighting aperture must be adapted to the objective aperture.
Moreover, the image contrast is dependent upon the size of the illuminated object field. This field should not be larger than the object field recorded by the objective, in order to avoid the negative influences due to stray light or an irradiation.
The object field is proportional to the magnification of the objective employed. For this reason, upon each change of objective an appropriate adaptation of the light field diaphragm bounding the illuminated object field must also take place.
A regulation of the total brightness cannot take place by a change in size of the aperture diaphragm or light field diaphragm, for the above-indicated reasons, but must be undertaken independently thereof by either gray filters or regulation of the lamp voltage.
In conventional lighting devices for microscopes, halogen lamps are used, which usually exhibit a coil surface in the aspect ratio of 2:1. This rectangular lighting field can be brought, using an appropriately shaped concave mirror, to a square coil image of the light source and can be imaged using Kohler lighting into the circular aperture diaphragm.
When using objectives with small apertures or large fields, there is the difficulty that the light intensity decreases towards the margin of the lighting field in accordance with the cosine law, while when using objectives with large apertures or small fields the size of the imaged lamp coil is not sufficient for the illumination of the aperture diaphragm. This leads to uneven illumination in the plane of the aperture diaphragm.
According to AT-PS 183 246, the problem associated with objectives with large apertures or small fields was solved by scattering image of the lamp coil by means of a ground glass plate introduced into the lighting beam path. Using this arrangement, a large aperture diaphragm can be illuminated uniformly by a small field. However, the disadvantage of this method is that uncontrolled stray light in the lighting beam path passes into the object plane and which decreases the contrast and the resolving power of the entire microscope system.
A lighting device for optical systems which overcomes this disadvantage is disclosed in DE 41 02 507. Through a deflector plate, disposed in the lighting beam path, with a plurality of individual prisms uniformly distributed over the entire surface, a uniform illumination of the aperture diaphragm is achieved such that the light source is optically sharply imaged with a plurality of secondary images into the plane of the aperture diaphragm.
With this lighting device, however, the cosine law is not taken into consideration. As a result of the reflected ray angle, a decrease in brightness takes place in the plane of the lighting field diaphragm towards its margin. When using large fields, i.e., a large collector lens diameter and small apertures, no uniform illumination of the plane of the lighting field diaphragm is achieved.